INDUCTION OF A NEW LIPOXYGENASE FORM IN CUCUMBER LEAVES BY SALICYLIC-ACID OR 2,6-DICHLOROISONICOTINIC ACID

Changes in lipoxygenase (LOX) protein pattern and/or activity were inv estigated in relation to acquired resistance of cucumber (Cucumis sati vus L.) leaves against two powdery mildews, Sphaerotheca fuliginea (Sc hlecht) Salmon and Erysiphe cichoracearum DC et Merat. Acquired resist ance was established by spraying leaves with salicylic acid (SA) or 2, 6-dichloroisonicotinic acid (INA) and estimated in whole plants by inf ested leaf area compared to control plants. SA was more effective than INA. According to Western blots, untreated cucumber leaves contained a 97 kDa LOX form, which remained unchanged for up to 48 h after patho gen inoculation. Upon treatment with SA alone for 24 h or with INA plu s pathogen, an additional 95 kDa LOX form appeared which had an isoele ctric point in the alkaline range. For the induction of this form, a t hreshold concentration of 1 mM SA was required, higher SA concentratio ns did not change LOX-95 expression which remained similar between 24 h and 96 h but further increased upon mildew inoculation. Phloem exuda tes contained only the LOX-97 form, in intercellular washing fluid no LOX was detected. Immunocytochemical localization revealed LOX protein in the cytosol of the mesophyll cells without differences between the forms.